Like most other littorinid snails, Echinolittorina placida is unable to crawl or feed on sand or mud, and therefore requires hard substrates such as rock outcrops or man-made structures for existence. The natural coastline of the Gulf of Mexico consists almost entirely of sand and mud - only in the southern part (Yucatan Peninsula and as far west as Veracruz) are there outcrops of rock.
The natural distribution must therefore have been restricted to this part of the present range. Since the late nineteenth century, numerous jetties and seawalls have been constructed along the Gulf coast, and this has permitted the enormous expansion of the range of Echinolittorina placida.
This species colonises by means of its pelagic eggs and larvae, that live in the water column for an estimated 3 weeks and can potentially drift for several hundred kilometres in that time.
During the twentieth century, local biologists and collectors have noted the arrival of this common and conspicuous snail on the jetties of the Gulf coast. It was first found at Sabine Pass, Texas in 1950, then in Louisiana in 1960, Panama City, Florida in 1971, and reached the eastern Atlantic coast of Florida in 1985. A population was found on an isolated rock outcrop in North Carolina in 1995, making a total range extension along 4500km of coastline in about 100 years.
This is yet another example of unintentional human introduction of alien species, but through the unusual mechanism of provision of new habitat where none existed before.
The inferred original distribution of Echinolittorina placida indicates that it is a tropical species, so it is remarkable that it can survive in the temperate waters of North Carolina. Whether climate change has contributed to its northward spread is unknown.
The snails live in crevices and among barnacles high in the intertidal zone, and can be found on a variety substrates:
They reach largest size on limestone rocks, which provide optimal crevices and microalgal food. They occur mainly in sheltered situations where wave action is not strong and where nutrient levels in the water are high, for example in lagoons, estuaries and behind breakwaters. On the Atlantic coast of Florida, for example, they do not occur on the exposed, oceanic, open coast, but only in sheltered inlets - this is the origin of the name placida, meaning quiet or still.
These snails occur in enormous numbers on suitable surfaces and are resilient to unfavourable conditions such as long periods of drying or rainfall. Their tolerance of pollution has not been studied, so the effects of recent oil pollution in the Gulf of Mexico are unknown.